The present invention generally relates to liquid crystal display driving circuits and liquid crystal display units, and more particularly to a liquid crystal display driving circuit which drives a liquid crystal display panel with lower power consumption and to a liquid crystal display unit having such a liquid crystal display driving circuit.
When a D.C. voltage is continuously applied to a liquid crystal display (LCD) panel of a LCD unit, a phenomenon called burn-in occurs whereby displays remain on the LCD panel. For this reason, an operation which is sometimes referred to as an A.C. type drive is carried out to invert the polarity of the voltage applied to the LCD panel at a predetermined period.
The A.C. type drive can generally be categorized into a method which inverts the polarity of the driving voltage with respect to a voltage of a common electrode, and a method which periodically inverts the voltage of the common electrode. In this specification, the former method will be referred to as a fixed drive method, and the latter method will be referred to as an inversion drive method, for the sake of convenience.
According to the fixed drive method, a voltage of 0 V is applied to a common electrode of the LCD panel, and a data electrode is driven by a voltage having both polarities such as +5 V and -5 V, as shown in FIG. 1A. As a result, an A.C. type signal shown in FIG. 1B is obtained. However, because the data electrode is driven by the voltage having both the positive and negative polarities, the construction of a LCD driving circuit becomes complex.
On the other hand, according to the inversion drive method, the common electrode of the LCD panel is driven as shown in FIG. 2A. As a result, an A.C. type signal shown in FIG. 2B is obtained. However, because the common electrode is driven, the power consumption is large due to the large load which is driven.
Equipment which use the LCD unit are generally driven by batteries, and it is desirable to reduce the power consumption of the LCD unit in order to extend the serviceable life of the batteries. For this reason, most of the conventional equipments which use the LCD unit employ the fixed drive method described above. But in order to reduce the cost of the LCD unit and to further reduce the power consumption of the LCD unit, it is desirable to simplify the construction of the LCD driving circuit.
According to the inversion drive method described above, a power P which is consumed to simply drive the common electrode can be obtained from the following formula, where f denotes a frequency of the A.C. type signal which is 30 kHz, V denotes a voltage of 5 V, and Cp denotes a total capacitance of the common electrode. EQU P=Cp.multidot.V.sup.2 .multidot.f=0.5(.mu.F).multidot.5(V).multidot.5(V).multidot.30(kHz)=375(mW )
Generally, the total power consumption of the LCD driving circuit described above is on the order of 1 W, for example. Hence, the power P that is consumed to simply obtain the A.C. type signal accounts for approximately 40% of the total power consumption of the LCD driving circuit.
Therefore, the conventional LCD driving circuit had a problem in that a large portion of the total power consumption of the LCD driving circuit is used up in order to obtain the A.C. type signal.
In order to reduce the power consumption of the LCD driving circuit, that is used up to obtain the A.C. type signal, it is conceivable to reduce the total capacitance Cp of the common electrode, the voltage V and the frequency f of the A.C. type signal.
The total capacitance Cp of the common electrode is dependent on the characteristics and size of the LCD panel. But recently, the size of the LCD panel has become relatively large and the internal structure of the LCD panel has become more miniaturized and complex. For this reason, the total capacitance Cp of the common electrode has a tendency of increasing with the increased size and complexity of the LCD panel, and it is difficult to reduce the total capacitance Cp of the common electrode.
The power P that is consumed in order to obtain the A.C. type signal is proportional to the square of the voltage V. For this reason, if the voltage V is reduced from 5 V to 3.3 V, for example, it would be possible to reduce the power consumption P to approximately one-half. However, it is difficult to greatly reduce the voltage V in actual practice due to various restrictions posed by circuit parts of the LCD driving circuit other than the circuit part which obtains the A.C. type signal, and there is a limit to greatly reducing the voltage V.
In addition, the frequency f of the A.C. type signal has already been reduced close to the limit in order to prevent the burn-in described above and the flicker that is visible to the human eyes. Thus, it is extremely difficult to further and considerably reduce the frequency f.
Therefore, it was difficult to reduce the power that is consumed in order to obtain the A.C. type signal in the conventional LCD driving circuit, and it was impossible to greatly reduce the power consumption of the LCD driving circuit.